That little part of your brain that will hear or think of something hilarious but inappropriate, which will act as its ignition. It is then powered by your attempts to ignore it, and will accelerate, never reaching a terminal speed and only stopping when you blurt out whatever thought started it at the worst possible moment. It is related - but not linked to - that part of your brain that inexorably broadcasts horrific images to the rest of your brain at inconvenient times, for example: images of corpses while eating, images of your mum during sex etc.

Called 'man motor' because there is no evidence of this phenomenon ever occurring in females.

Your idiot thoughts during a funeral: Hey, that dude who was killed in a horrific car accident at the age of 25's wife's face kinda looks like a pan-fried vagina.